Mrs Conway — a named character who doesn't appear elsewhere in the novel. (("... Actually it's a mistake. It's supposed to be Mrs Conroy. But the names got mixed up some way or other. It was supposed to be a nod to Strong, of gratitude and respect" – JO'N.)) Mrs Conroy is a character in L.A.G Strong's semi-autobiographical novel, The Garden, 1931, which is also set in Glasthule, and at the same period. She is the grandmother of Dermot, the focal character.

left foot — here the meaning is "Protestant". Either a mistake, or anachronistic; or both. …

dig with the left foot: "be a Catholic" – Routledge Slang. ("from the Northern Irish saying that farm workers in Eire use the left foot to push a spade when digging" – Collins.) dig with the right/left/other/wrong foot: "sectarian characterisation of an individual of another religious persuasion (gen. Catholic / Protestant) Ulster – Slanguage.
cf. kick with the wrong foot: "to be of the opposite religion to that which is regarded as acceptable or to that of the person who is speaking" Scot. and Irish – Collins. left-legger is sometimes used by Irish Catholics to describe a Protestant, from the belief that Protestants genuflect with the left leg, whilst RCs genuflect with the right.
None of these idioms seems to date much earlier than the 1940s. Left-handedness, of course, has long been a designation of gaucheness and sinisterity.

bagmen — commercial travellers. ((pejorative and colloquial – Partridge.)) But cf. (("... a bagman, as they call that sort of globe-trotting fellow that knocks about from one place to another, and takes all the fun he can out of it at other people's expense" – Somerville & Ross, All on the Irish Shore, 1903.))

go-boys — opportunists. ((go-boy: "juvenile delinquent, chancer" – Slanguage.)) (("worthless person, of no value to anyone, but who is not aware of this himself" – DHE.))

on the make — out for what they can get. (("intent on booty or profit" – Partridge HS.))

on the cadge — looking for free drinks. ((cadge: "the act or the practice of begging" low colloquial – Partridge HS.)) (("For example, a waiter when hanging about for 'a tip' is said to be cadging or 'on the cadge'" – Farmer 1904.))

decent skin — a good generous person. (("decent oul' skins; never see you caught for the price of a smoke, a read of the paper or the loan of a pump for your bike..." Slanguage.)) (("to say that a person was 'a dacent poor skin' was the highest praise" – Máirín Johnston, Around the Banks of Pimlico, 1985.))

no more side than a margarine — ? very unpretentious. ((side: "conceit, swagger, pretentiousness" – Partridge HS.)) Margarine – ? perhaps from margarine, a lowly substitute, having no pretensions to being butter. Except that it has.

could not engage — could not be certain. (("'I'll engage you visited Peggy when you were in town': i.e. I assert it without much fear of contradiction: I warrant" – PWJ 1910.))

rise — ((take a rise out of (one): "to raise a laugh at, by some form of pretence or dissimulation" – OED.))

consumptive — affected by Consumption, an old term for tuberculosis (TB).

with the Colours — serving with a regiment. ((colours: "a pair of silk flags borne by a military unit, esp. British, comprising the Queen's Colour showing the unit's crest, and the Regimental Colour showing the crest and battle honours" – Collins.)) (("I'm as staunch a Britisher as you are, sir. I fought with the colours for king and country" – Joyce, Ulysses, 1922.))

segotia — old friend. (("term of endearment" origin obscure; "it has been suggested that it derives from a corruption of Irish seo dhuitse = 'here it is you are'" – DHE.))

may your hand be stretched ... ex Irish proverbial (("May your right hand always be stretched out in friendship, and never in want" – Paul Dickson, Over 1,500 of the Best Toasts, etc., 1981.)) ... and never your neck — punning stretch "to reach out" and ((stretch: "to be hanged" obsolete – OED.))

price of a skite — the cost of having a few drinks. ((skite: "a jollification, a spree" – OED.)) ((on the skite: "engaged in serious drinking" – Slanguage.))

peelers — policemen. (("after Sir Robert Peel, as founder of the Irish constabulary, 1817" – Partridge.)) cf. "Bobby", the usual nickname for a policeman in England, after the same Sir Robert Peel, who in 1829 founded the London Metropolitan Police.

nabbed him — arrested him. ((low slang – Partridge HS.))

recruitment poster — recruitment poster for the British Army, defacement of which was a serious crime under the wartime Defence of the Realm Act, 1914.